On the Occurrenceof Pelagic Tunicates (Thaliacea)]In the Waters of the Etlglish Ohannel Off Plymouth
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On the Occurrenceof Pelagic Tunicates (Thaliacea)]in the Waters of the Etlglish Ohannel off Plymouth. By F. S. Russell, D.S.C.,B.A., Naturalist at the Plymouth Laboratory, and Anna B. Hastings, M.A., Ph.D., British Museum (NaturalHistory).
IT is only occasionally that pelagic tunicates of the genera Doliolum and Salpa are recorded in numbers in the waters off Plymouth. In view of their possible use as indicators of the presence of Atlantic water (see Bowman, 1923; and Schmidt, 1909, p. 159), it is of value that their occurrence should be put on record. Regular weekly catches of plankton have been made since 1930 with half-hour oblique hauls with the two- metre stramin ring-trawl. During this period species of both genera have appeared in such numbers that it has seemed desirable to make a careful examination of the species occurring.
Doliolids. PREVIOUSRECORDS. Doliolum* was found in considerable numbers in tow-nettings at Plymouth in August and the beginning of September, 1893,soon becoming scarce (Garstang, 1894,p. 222; Browne, 1896, p. 171). In 1895Doliolum was exceedingly abundant at the beginning of September and there was a gradual decrease towards the end of the month (Browne, 1896, p. 171). In September, 1897, Doliolum was not found (Browne, 1898, p. 191). The only other record in the Plymouth Marine Fauna (1931) is of several specimens of D. nationalis taken in November, 1904. Salps. Large shoals of Salpa democraticavisited the Sound from the middle of June to the end of the first week in July, 1893 (Garstang, 1894, p. 222, Thalia democratica-mucronata). In September, 1893 and 1897, none were taken and in September, 1895, a few on the 9th only (Browne, 1896, p. 171; 1898, p. 191). Salpa fusiformis was taken in the region of the Eddystone in August, 1901, and July and September, 1925 (Plymouth Marine Fauna, 1931). * Specimens of D. nationalis recorded as D. tritonis, see Fowler, 1898, p. 593. 636 F. S. RUSSELL AND ANNA B. HASTINGS.
It was also present in large numbers in the Sound in September, 1917, but no collections were available from offshore waters. Salpa zonaria was taken in numbers by Professor J. H. Orton when on a cruise in s.s. Oithona in September, 1914, off Falmouth Bay. We are indebted to Professor Orton for allowing us to examine these specimens.
RECENT OBSERVATIONS. In Table I are given records of the numbers of tunicates taken in the weekly oblique hauls in 1931 and 1932. Throughout the whole of 1930 none were seen, and in Table I only those months in which specimens were seen are inCluded. TABLE 1.
NUMBERS OF DOLIOLIDS AND SALPS IN HALF-HOUR OBLIQUE HAULS WITH THE 2-METRE STRAMIN RING-TRAWL.
1931 Doliolum. S. mucronata. S. fusiformis. Sept. 10th 3 solitary 2 aggregate ,,24th 21 Oct. 1st 60 20 aggregate " 8th 798 " 15th 1513 " 22nd 515 " 29th* 1412 1932 Aug. 26th 18 aggregate. Sept. 1st J 47 aggregate. \..14 solitary. " 7th 103 aggregate. { 4 solitary. " 14th 32 J 696 aggregate. \.. 6 solitary. " 21st 199 J 311 aggregate. \..199 solitary. " 29th 213 116 aggregate. Oct. 4th 164 J 1330 aggregate. \.. 120 solitary. 11th 1722 " 19th 3450 1 aggregate. " 26th 2357 Nov. 14th 331
* The next haul was on November 25th, when none:were seen. PELAGIC TUNICATES OFF PLYMOUTH. 637
. The Table shows that the autumn of 1931was notable for the appearance of Doliolids and a few S. mucronata,. and that 1932 is notable for the occurrence of Doliolids and also of Salpa fusiformis which has appeared in numbers greater than in living memory of any member of the Plymouth Laboratory staff, except perhaps for the year 1917 when it invaded the Sound and Hamoaze. Examinations were also made of ring-trawl catches during the following periods, but no Doliolids or Salps seen-May to July, 1924; April to August, 1925; April to September, 1926; April to December, 1927; January to April, 1928; April to October, 1929. (For dates of collections, see Russell, 1930, p. 714.) Two species of Doliolum were taken in large numbers in 1931, namely D. nationalis and D. gegenbauri. In 1932, however, it is to be noted that only D. nationalis was seen. Table II shows the composition of samples of some of the catches.
TABLE II.
COMPOSITION OF SAMPLES OF DOLIOLUM CAUGHT ON THE DATES GIVEN. 8.10.31. 15.10.31. 22.10.31. 29.10.31. D. nationalis f (phorozooids) 84 103 7 51 - - - L (gonozooids) 5 D. gegenbaurif(phorozooids) 17 17 4 24 L(gonozooids) 21 ' 40 12 76 D.sp. (old blastozooids) 64 15 11 49 D. ? nationalis (blastozooids) - - - 1
14.9.32. 21.9.32. 26.9.32. 29.9.32. 11.10.32. 19.10.32. 26.10.32 D.nationalis 13 83 311 72 646 303 388 (phorozooids)
THE SPECIES OF THALIACEA OCCURRING IN THE ENGLISH CHANNEL OFF PLYMOUTH.
(1) Salpafusiformis Cuvier, 1804, p. 382, PI. LXVIII, Fig. 10. The specimens of S. fusiformis obtained in 1932 have the smooth test of the typical form. The relationship of this form to S. fusiformis aspera (Cham.) (=S. echinata Herdman, see Ihle, 1912, p. 40) is still debatable. Ritter (1905, p. 69) maintained that the characters of aspera appeared in older specimens. Apstein (1906, p. 268) regarded it as the cold water form. Examination of the material in the British Museum has given no conclusive evidence. ~
638 F. S. RUSSELL AND ANNA B. HASTINGS.
(2) Salpa democratica Forskal, 1775, p. 113. (3) Salpa zonaria (Pall.). Holothurium zonarium Pallas, 1774, p. 26. PI. I, fig. 17 A, B, C. Only recorded off Falmouth Bay. (4) Doliolum nationalis Borgert, 1893, p. 406, text-fig. (5) Doliolum gegenbauri Ulianin, 1884, p. 134, PI. VII, fig. 5. No member of the group of species to which D. gegenbauri belongs had previously been recorded at Plymouth, for Garstang's record of D. tritonis was based on material of D. nationalis (see above). D. gegenbauri and D. tritonis are very closely allied and may yet prove to be synonymous. The gonozooids are distinguished by the stigmata, which end ventrally at the 5th muscle band in D. gegenbauri and in D. tritonis extend a little further forward to end between the 4th and 5th bands. Except in a few badly crushed specimens, where the position of the stigmata could not be made out at all, the Plymouth material was all clearly of the gegen- bauri type. 6) Blastozooids of Doliolum. The blastozooids obtained in 1931 are nearly all too old for identifica- tion. One, however, has the stigmata and gut complete, though the ~tigmata are a little crushed so that their exact shape cannot be deter- mined. It agrees closely with the blastozooid of D. denticulatum* (Grobben, 1882,PI. I, figs.3-5; for synonymy see Neumann, 1906,p. 222), no difference being apparent except that the endostyle extends a little further back, reaching the 5th muscle band. D. denticulatum does not occur at Plymouth, but its nearest ally, D. nationalis, together with D. gegenbauri, was in the haul in which this one blastozooid was taken. Fowler (1905, p. 93) had exactly similar blastozooids, some of which are in the British Museum (Nat. Hist.). He attributed them very tentatively to D. tritonis, recognising D. nationalis as the only alternative. Ritter (1905, p. 86) has shown reasons for connecting a different blastozooid with D. tritonis. In it the intestine is short and slightly twisted, the anus presumably being fu~ther forward than in Fowler's blastozooid, in which the intestine is straight and reaches the 8th band. The otocyst is in the third intermuscular space in both, but in Ritter's is nearer the 3rd, in Fowler's nearer the 4th band. Despite the fact that D. gegenbauri is the classical material for the study of the life-history of Doliolum, there appears to be no precise information about the characters of the intestine
* Ritter (1905, p. 93) gives no reasons for his conclusion that this blastozociid belongs, not to D. denticulatum, but to some unknown gonozooid. The name D. ehrenbergii Ulianin belongs to a gonozooid (which may not exist) with the stigmata reaching the first muscle band. Ritter's use of the name for his blastozooid would only be justified if he had proved that it developed from a gonozooid of that type. .His references to gegenbauri in this paragraph must be read as ehrenbergii. PELAGIC TUNICATES OFF PLYMOUTH. 639 of the blastozooid, except that the anus lies between the 7th and 8th bands (Neumann, 1906, p. 219). There is thus some reason for distinguish- ing Fowler's blastozooid with its straight intestine and anus under the 8th band from both D. tritonis and D. gegenbauri, whether they are synonymous or not, and for concluding that it belongs to D. nationalis.
DISTRIBUTION.
All the five species of Thaliacea recorded above are widely distributed in the warmer seas of the world. The Salps are all found in the Atlantic, Indian, and Pacific Oceans. Apstein (1906, pp. 261-277) states that S. democraticais the commonest Salp in warm water, and that it is carried northwards in the Gulf Stream; that S. fusiformis is one of the most frequent Salps, above all at home in warm water but occurring in higher latitudes; and that S. zonaria is found above all in the warm parts of the oceans but is carried north in the Gulf Stream. Of all the warm-water species of Salps, these three especially appear to be able to withstand a considerable lowering of temperature, being carried north into temper- atures of 10° C. in waters north of Scotland. Neumann (1913,p. 230-231) states that D. nationalis and D. gegenbauri occur in the Atlantic and Indian Oceans and in the Mediterranean. They however appear to be more confined to the warm water than do the above species of Salps. It is hoped that when future records are available it will be possible to indicate to what extent the presence of these tunicates in English Channel waters may be due either to movement of Atlantic water or to high temperatures allowing successful reproduction and survival in the area.
REFERENCES.
ApSTEIN,C. 1893. Die Thaliacea der Plankton-Expedition. B. Ver- theilung der Salpen. Hensen, Ergebn. Plankton. Exp. II E. a.B., pp.68. - 1906. Salpen der Deutschen Tiefsee-Expedition. Wiss. Ergebn. Deutsch. Tiefsee-Exped. XII, pp. 247-290. BORGERT,A. 1893. Uber Doliolum denticulatum und eine neue dieser Art nahe verwandte Form aus dem atlantischen Ocean. Z. wiss. Zool. Leipzig, LVI, pp. 402-408. BOWMAN,A. 1923. The Biological Interchange between the Atlantic and the North Sea. Rep. Brit. Assoc. Adv. Sci., London, 1922, p.367. 640 F. S. RUSSELL AND ANNA B. HASTINGS.
BROWNE,E. T. 1896. On the Changes in the Pelagic Fauna at Plymouth during September, 1893 and 1895. Jour. Mar. BioI. Ass., N.S., Vol. IV, pp. 168-173. - 1898. On the Pelagic Fauna at Plymouth for September 1897. Jour. Mar. BioI. Ass., N.S., Vol. V, pp. 186-192. CUVIER,G. 1804. Sur les Thalides (Thalia, Brown), et sur les Biphores (Salpa, Forskaohl). Ann. Mus. Rist. nat. Paris, IV, pp. 360-382. FORSKAL,P. 1775. Descriptiones Animalium, pp. 164. FOWLER, G. H. 1898. Contributions to our Knowledge of the Plankton of the Faeroe ChanneL-No. VI. Proc. Zool. Soc. London, pp. 567-585. - 1905. Biscayan Plankton collected during a Cruise of R.lVLS Research, 1900. IV. Thaliacea. Trans. Linn. Soc. London, Zool. X, pp. 89-101. GARSTANG,W. 1894. Faunistic Notes at Plymouth during 1893-4. Journ. Mar. BioI. Ass., N.S., Vol. III, pp. 210-235. GROBBEN,C. 1882. Doliolum und sein Generationswechsel. Arb. zool. Inst. Wien IV hft. 2,pp. 77-98. IHLE, J. E. W. 1912. Salpre I. Desmomyaria. Schulze, Das Tierreich lief. 32, pp. i-x, 1-67. NEUMANN,G. 1906. Doliolum. Wiss. Ergebn. Deutsch. Tiefsee-Exped. XII, pp. 95-243. - 1913. Die Pyrosomen und Dolioliden der Deutschen Siidpolar- Expedition 1901-1903. Ergebn. Deutsch. Siidpolar-Exped., XIV, pp. 1-34. PALLAS,P. S. 1774. Spicilegia Zoologica, I, fasc. 10, pp. 41. RITTER,W. E. 1905. The Pelagic Tunicata of the San Diego Region, excepting the Larvacea. Univ. Calif. Publ. Zool. II, pp. 51-112. RUSSELL,F. S. 1930. The Seasonal Abundance and Distribution of the Pelagic Young of Teleostean Fishes caught in the Ring-Trawl in Offshore Waters in the Plymouth Area. Journ. Mar. BioI. Ass., N.S., Vol. XVI, pp. 707-722. SCHMIDT,J. 1909. The Distribution of the Pelagic Fry and the Spawn- ing Regions of the Gadoids in the North Atlantic from Iceland to Spain. Rapp. Cons. Explor. Mer. Copenh. X, No.4, pp. 1-229. ULIANIN,B. 1884. Die Arten der Gattung Doliolum im Golfe von Neapel. Fauna u. Flora des Golfes von Neapel, X, pp. 140.